The present invention generally relates to an apparatus for manufacturing electronic devices, and more particularly to a wafer holder that supports stably a semiconductor wafer during a chemical-mechanical planarization process.
During manufacture of integrated circuit wafers, the wafer surface is sometimes treated by a chemical-mechanical planarization (xe2x80x9cCMPxe2x80x9d) process. The treatment, also called chemical-mechanical polishing, is carried out by a so called CMP machine.
An example of a typical CMP machine is described in U.S. Pat. No. 5,702,292 to Brunelli et al. During polishing the CMP machine brings the wafer surface in contact with a rotating polishing pad under a biasing force.
Some CMP machines maintain the wafer in a wafer holder that faces up for loading the wafer on it. The wafer holder then faces down for polishing the wafer. During the reorientation the wafer is prevented from falling off by using a vacuum, as is taught in U.S. Pat. No. 5,095,661 to Gill et al.
Referring to FIGS. 1A and 1B, the wafer is preferably maintained in a recess of the wafer holder, as is also taught in U.S. Pat. No. 5,597,346 to Hempel, Jr. Specifically, a wafer holder 300 defines a recess 302 for receiving therein a wafer 304. The recess is bounded by a rim that decreases in height away from the recess.
During polishing, the biasing force enhances friction, which causes the wafer to shift laterally within the recess, in spite of the vacuum. Shifting is unavoidable, because the diameter of the recess must be larger than that of the wafer, so that the recess can receive the wafer in the first place. The diameter difference xe2x80x9cdxe2x80x9d determines the extent of shifting.
The lateral shifting results in uneven polishing, which limits how well the wafer can be polished. Uneven polishing results in local areas of over-polishing and under-polishing, that interfere with photolithographic etching processes for making integrated circuit structures. It also results in an uneven thickness of the planarization layer of the wafer. This does not permit a good functional die to be achieved from the wafer, and makes it difficult to maintain fine resolution tolerances in the wafer.
Accordingly, an object of the present invention is to provide a wafer holder for chemical-mechanical planarization machines that supports stably a semiconductor wafer, effectively preventing it from shifting during polishing.
In order to accomplish the above object, the present invention provides a clamping wafer holder for chemical-mechanical planarization machines. The clamping wafer holder comprises a plate having a surface for receiving on it the wafer, and a retainer around the surface. The retainer includes at least two jaws shaped and arranged such that they define a recess with the receiving surface. The wafer is to be placed in the recess.
The wafer holder also includes an actuator coupled with the retainer and adjusting it. The retainer can be adjusted from an open position where the jaws are separated from each other, to a closed position where the jaws clamp an edge portion of a wafer in the recess. When the retainer is in the closed position the jaws preferably contact each other. When they do, they define a continuous inner surface.
In the first embodiment, the inner surface is cylindrical. In the second embodiment the surface includes a stopper that engages a flat zone of a wafer. Where the shape of the jaws does not match exactly the periphery of the wafer, elastic inserts are mounted on the jaws that result in distributing the lateral clamping force more evenly.
A vacuum source is coupled with the plate, to hold the wafer in the recess during reorientation. The actuator is advantageously operated by the vacuum source. The actuator is coupled with at least one of the jaws by three pins. The actuator is preferably coupled also with the plate, in which case the pins go through elongate slots in the plate.
A method for performing chemical-mechanical polishing on a surface of a wafer according to the invention comprises the step of placing the wafer in a wafer holder such that the wafer surface is exposed. A vacuum helps maintain the wafer onto the wafer holder.
The wafer is then laterally stabilized with respect to the wafer holder. This is performed by radially clamping an edge portion of the wafer. Clamping is performed by moving jaws of a retainer of the wafer holder. Then the exposed wafer surface is positioned against a polishing surface, and polished as is known in the art.
The wafer holder of this invention results in a configuration that eliminates the diameter difference xe2x80x9cdxe2x80x9d of the prior art. The wafer is supported stably and prevented from shifting laterally during polishing. This effectively reduces the amount of uneven polishing, thus achieving a good wafer. In addition, when the retainer includes a stopper that engages the flat zone of the wafer, the wafer holder does not need a device for adjusting the weight balance. Thus, the wafer holder is simpler to make.